Joining Si wafers together directly without sandwiching an insulator film such as an oxide film causes considerable generation of voids in the junction plane. There is a fact that bonding wafers together either or both of which is oxidized and has an oxide film with a certain thickness or more can produce less voids than the case of bonding wafers directly. However, the method is a production method of the so-called SOI (Silicon On Insulator) wafers, and use of the method leaves a buried oxide film at the junction interface. Consequently, a direct bonded wafer cannot be produced.
On the other hand, a method for producing a direct bonded wafer is disclosed in Japanese Unexamined Patent Application Publication No. 2000-36445. In the method, wafers with a native oxide film are bonded together at room temperature and undergo a heat treatment under an atmosphere except oxidizing atmosphere to diffuse and remove the oxide film layer at the bonding portion. However, even use of this method cannot reduce the void count enough.
What is mentioned above is summarized as follows. Firstly, direct bonding causes considerable generation of voids. Secondly, bonding after growing an oxide film reduces generation of voids, but leaves a buried oxide film. And, a native oxide film cannot prevent voids sufficiently. Consequently, it has been difficult to produce a direct bonded wafer in which voids are reduced sufficiently.
Furthermore, reducing voids requires reduction of particles and so on at the junction interface, and uniform conditions of the junction plane. However, either requirement does not meet production level currently.